1. Field of the Invention
The present invention relates to a high-purity ferroboron used as a raw material for an amorphous alloy or the like, and a mother alloy for an iron-base amorphous alloy using said ferroboron, an iron-base amorphous alloy by using said mother alloy, and methods for producing the same.
2. Description of the Related Art
An amorphous alloy is excellent in magnetic and mechanical properties and, as such, is viewed as a promising material for various industrial applications. An iron-base amorphous alloy, for example an amorphous alloy of an Fe—B—Si or Fe—B—Si—C system, is used, in particular, as a material for the iron core of a power transformer, a high frequency transformer and the like because it has a low core loss, a high saturation magnetic flux density and a high magnetic permeability.
An amorphous alloy of this kind is produced by rapidly cooling and solidifying a mother alloy in a molten state through a single-roll process, a twin-roll process or the like. Such a process is one wherein molten metal is rapidly solidified by spraying it through an orifice or the like onto the outer surface of a rapidly rotating metal drum and, by so doing, a thin strip or a fine wire is cast.
A mother alloy is an alloy the chemical composition of which has been adjusted to that of the amorphous alloy. In the case of an iron-base amorphous alloy as mentioned above, a mother alloy is produced by blending ferroboron, a diluent base iron, and auxiliary materials such as Si and C and, by so doing, adjusting the composition.
If impurities are contained in a mother alloy, an amorphous structure is not stably formed during rapid-cooling solidification and, as a consequence, excellent properties are not obtained. For this and other reasons, all the materials used as the raw materials for a mother alloy have been high-purity ones; electrolytic iron has been used as the diluent base iron.
Ferroboron has been produced in a melting-and-reducing furnace such as an electric furnace or the like using a boron source such as boron oxide, boric acid or the like, a base iron and a carbon-base reducing agent such as coke, charcoal, powdered coal or the like as the raw materials. In particular, electrolytic iron has been used as a base iron for high-purity ferroboron.
The content of boron in an iron-base amorphous alloy is several mass %, and two methods have been proposed as the production methods of the mother alloy thereof: a method of diluting ferroboron having a boron content of not less than 10 mass %, the ferroboron being produced through an electric furnace process; and the other method of finely adjusting the composition of ferroboron having a boron content of several mass %, the ferroboron being produced in a shaft furnace or a ladle refining facility. What is actually employed is the former method. The main reason for this is that the former method entails a high boron utilization efficiency and a low cost. Another reason is that, by the former method, the content of C can be lowered by increasing the content of B.
The solubility of C in ferroboron is in inverse correlation with the content of B: the lower the content of B, the higher the solubility of C. Therefore, in the case where C is harmful as an impurity, increasing the content of B is an effective measure for reducing the amount of C.
Japanese Unexamined Patent Publication No. S59-232250 discloses the above inverse correlation and a technology that makes it possible to commercially produce ferroboron having a B content of not less than 10 mass % and a C content of not more than 0.5 mass %. However, a problem in the technology disclosed therein is that, when ferroboron having a high boron content is produced through an electric furnace process, the electric power consumption rate is high.
In addition, Japanese Unexamined Patent Publication No. S59-126732 discloses a method of decreasing the content of C by bubbling oxygen gas through molten ferroboron. However, a problem of the method disclosed therein is that boron is also oxidized by the oxygen gas and therefore the boron utilization efficiency is decreased.
Further, as a production method for low-Al, high-purity ferroboron, Japanese Unexamined Patent Publication Nos. S59-232250 and 560-103151 disclose methods for obtaining ferroboron having a B concentration of 10 to 20 mass % using an electric furnace. However, when scrap iron is used as the base iron in each of the proposed methods, the concentration of Al contained in the scrap iron fluctuates and, as a consequence, the guaranteed Al content is less than 0.20 mass %. However, up to the present, commercially available low-Al, high-purity ferroboron has required a guaranteed Al content of less than 0.025 mass % and, to cope with the requirement, electrolytic iron has been used as the base iron in each of the methods and thus the product has been expensive.
As means of obtaining ferroboron for an iron-base amorphous alloy at a low cost, some methods are disclosed wherein a melting-and-reducing process that does not use an electric furnace is employed, although the ferroboron obtained has a low B concentration. For example, Japanese Unexamined Patent Publication No. S58-77509 discloses a method for obtaining ferroboron having a B concentration of several mass % by reducing iron ore and boron oxide simultaneously in a shaft furnace and Japanese unexamined Patent Publication No. S58-197252 discloses another method for obtaining ferroboron having a B concentration of several mass % by adding boron oxide and a reducing agent to molten steel and reducing the boron oxide in a ladle refining furnace.
In these methods, however, unreduced boron oxide remains in slag and, as a result, the boron utilization efficiency is low. Boron oxide is a comparatively expensive raw material, and therefore these methods entail a rather high cost. What is more, as the environmental regulations have been tightened in recent years, the methods have come to bear an increased cost since the disposal of boron-containing slag entails a high treatment cost. Thus, while the proposed methods are considered effective for lowering the content of Al, the cost reduction, which has been the initial object of the inventions, is not achieved. For this reason, these methods are not commercially applied at present.
On the other hand, today's mass-produced steel is produced through a continuous casting process because the process has a high productivity and entails a low cost. Killed steel is applied in order to suppress gas generation in a continuous casting process. Al is generally used as a deoxidizing agent for mass-produced steel and, as a result, a considerable amount of Al is contained in the steel. For this reason, mass-produced steel has been considered unusable as a base iron for a mother alloy of an iron-base amorphous alloy and for the high-purity ferroboron used as a raw material for a mother alloy.
However, some of mass-produced steel has come to be produced by using Si and Mn as deoxidizing agents and, besides, thanks to the advancements of refining technologies, steel having a low Al content can be mass-produced even by using Al as a deoxidizing agent.
On the other hand, Japanese Unexamined Patent Publication Nos. H9-263914 and 2001-279387 disclose methods for producing an inexpensive mother alloy by using steel obtained through an ordinary steelmaking process as a diluent base iron, instead of using electrolytic iron that is expensive. In these technologies, the alloy contains, in mass, P: 0.008 to 0.1%, Mn: 0.15 to 0.5% and S: 0.004 to 0.05% as impurities and such a trace amount of contained P can prevent the properties of a cast thin strip from deteriorating even though Mn and S are contained to the extents of the amounts within the above ranges, respectively.
Further, Japanese Unexamined Patent Publication No. 2002-220646 discloses a method, that is applied to a thin strip after casting, of producing an iron-base amorphous alloy thin strip capable of exhibiting excellent magnetic properties and small variations thereof even when the temperatures at various portions of an iron core vary over a wide temperature range during the annealing of the iron core, by actively adding P to an amount within a specific range to an alloy having a chemical composition within a limited range. In this alloy too, Mn and S can be contained to the extents of the amounts in the above ranges, respectively, and, thus, ordinary steel can be used as the diluent base iron.